Electron optics for a minifying image tube

ABSTRACT

A minifying image tube, such as an X-ray converter or image intensifier tube, is disclosed employing a curved photocathode for converting a photon image into an electron image. A flat image converter screen is provided for converting the minified electron image into an optical output image. An electrode structure is disposed between the photocathode and the flat converter screen for accelerating and focusing the electron image upon the converter screen. The electrode structure includes a plurality of coaxially aligned axially spaced cylindrical electrodes of decreasing diameter taken in the direction from the photoemitter toward the converter screen. The final or anode electrode projects into a constricted electron exit aperture in the next preceding electrode structure in order to shape the equipotentials at the entrance of the anode such as to obtain uniform resolution of the electrode image focused upon the flat converter screen over substantially the entire area of the converter screen.

Levin et al.

[451 Aug. 8, 1972 [54] ELECTRON OPTICS FOR A MINIFYING IMAGE TUBE [72]Inventors: Nathan D. Levin, Los Altos Hills;

' Andreas Niewold, Sunnyvale; Wilfrid F. Niklas, Portola Valley, all ofCalif.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Oct. 3, 1969 [21] Appl. No; 863,508

[52] U.S. Cl. ..250/213 VT, 313/65 R [51] Int. Cl ..H01j 31/50 [58]Field of Search ..250/213 VT; 313/82 R, 65 R [56] References CitedUNITED STATES PATENTS 2,928,969 3/1960 Schneeberger ..313/65 3,026,4373/1962 Niklas ..313/65 3,303,345 2/1967 Wulms ..250/213 3,474,27510/1969 Stoudenheimer et al ..250/213 X Primary ExaminerWalter StolweinAttorney-Stanley Z. Cole and Gerald L. Moore A minifying image tube,such as an X-ray converter or image intensifier tube, is disclosedemploying a curved photocathode for converting a photon image into anelectron image. A flat image converter screen is provided for convertingthe minified electron image into an optical output image. An electrodestructure is disposed between the photocathode and the flat converterscreen for accelerating and focusing the electron image upon theconverter screen. The electrode structure includes a plurality ofcoaxially aligned axially spaced cylindrical electrodes of decreasingdiameter taken in the direction from the photoemitter toward theconverter screen. The final or anode electrode projects into aconstricted electron exit aperture in the next preceding electrodestructure in order to shape the equipotentials at the entrance of theanode such as to obtain uniform resolution of the electrode imagefocused upon the flat converter screen over substantially the entirearea of the converter screen.

ABSTRACT 4 Claims, 3 Drawing Figures PATENTEDAUB 8 m2 SHEET 1 0F 2 TTII.

INVENTORS NATHAN D LEVIN ANDREAS NIEWOLD WILFRlD FNIKLAS BY (3 M if MmATTORNEY PATENTEDAus' 8 I972 N wE IN'VENTORS NATHAN D. LEVIN ANDREASNIEWOLD WILFRID F. NIKLAS ATT ORNEY SHEET 2 or 2 cow N-@ MN m OE -'0 gzBY DESCRIPTION OF THE PRIOR ART Heretofore, rninifying image tubes,suchas image intensifier tubes and X-ray converter tubes, have employed asuccession of cylindrical focusing and accelerating electrodes disposedbetween the curved photocathode and a flat image converter screen.However, in these prior art tubes the final or anode electrode of thesmallest diameter had its electron entrance aperture or mouth portiondisposed downstream from the exit opening in the next precedingelectrode such that an axial gap was created between the upstream end ofthe anode and the downstream end of the next preceding electrode. Thiselectrode geometry results in distortion of the electron image asfocused upon a flat image converter screen such that resolution isdegraded near the outer perimeter of the image as compared to theresolution obtained near the center of the image. It is desired toobtain electron optics for such minifying image tubes which will yield auniformly high resolution over the entire image of the converter screen.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of improved electron optics for a rninifyingimage tube.

One feature of the present invention is the provision, in a minifyingimage tube, of an electron accelerating and focusing electrode structureincluding an anode electrode which projects at its upstream end into theexit portion of the next preceding electrode structure, whereby theequipotentials at the mouth of the anode are shaped to obtain nearuniform resolution of the electron image focused on a flat converterscreen over essentially all parts of the converter screen.

Another feature of the present invention is the same as the precedingfeature wherein each of the focusing and accelerating electrodes iscylindrical with the mouth portions of each successively smallerelectrode being disposed within the exit portion of the next precedinglarger electrode to define a series of convergent electron focusinglenses.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the electrode structure upstream fromthe anode has an exit portion with dimensions constricted substantiallyrelative to the dimensions of its upstream mouth portion.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectionalview of a minifying image tube incorporating features of the presentinvention,

FIG. 2 is an enlarged detail view of a portion of the structure of FIG.1 delineated by line 2-2 and depicting a prior art electrode structure,and

FIG. 3 is a schematic line diagram of a portion of the structuredelineated by line 3-3 of FIG. 1 and depicting the shapes of theequipotentials for the electrode structure of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a minifying image tube 1, in this case an X-ray converter tube,incorporating features of the present invention. The minifying imagetube 1 includes an evacuable envelope structure 2, as of glass, havingan outwardly domed spherical shaped face plate portion 3. A similarlydomed conductive electrode structure 5, which is transparent to an X-rayphoton image which passes through the face plate 3,. is mounted withinthe domed face plate 3. A scintillator layer 6 is deposited on theinside concave surface of the conductive electrode 5 and a photocathode7 is deposited over the inside surface of the scintillator layer 6. 1

X-ray photons passing through an object to be observed, pass through theface plate 3, conductive electrode 5 and are partially absorbed in thescintillator layer 6 to produce an optical photon image corresponding tothe X-ray image to be observed. The photon image is partially absorbedin the photocathode 7 to convert the photon optical image into anelectron image which is emitted into the evacuated envelope 2 from thephotocathode 7.

A flat image converter screen 8, made of a conventional fluorescentmaterial, is disposed at the end of a narrow neck portion of theenvelope 2 at the opposite end of the tube 1. A very thin conductivelayer 9, as of aluminum, is deposited over the converter screen 8 forapplying a uniform electrical potential to the screen 8.

An electrode structure 11 is disposed between the photocathode 7 and theimage converter screen 8 for accelerating and focusing the electronimages emitted by the photocathode 7 onto the image converter screen 8where an electron image is converted into an optical image forobservation or use. In a typical tube 1, the diameter of the imagefocused upon the image converter screen 8 is approximately one/tenth ofthe diameter of the electron image produced from the photocathode 7.Thus, the electrode structure 11 serves to minify the image obtainedfrom the photocathode 7 by a relatively large factor, such as by afactor of 10.

The electrode structure 11 includes a hollow cylindrical anode or fourthfocus electrode 12 disposed adjacent the image converter screen 8 andoperated at the same potential, as of about 25 kV positive with respectto the cathode, as that applied to the image converter screen 8 via theconductive electrode 9. The electrical potential for the anode 12 issupplied from a source of potential 13 via lead 14 and a feedthroughelectrode 15.

A third focus electrode 16 is disposed immediately upstream from saidanode electrode 12. The third electrode 16 includes a cylindrical mouthportion 17 which is constricted near its midpoint 18 and furtherconstricted at its downstream exit portion 19. The upstream end or mouthportion 21 of the anode electrode 12 is disposed within the exit opening19 in the third electrode 16. This geometry approximates that of a twocylinder electron lens and greatly facilitates obtaining uniformresolution of the electron image focused over the entire surface area ofthe image converter screen 8. More particularly, with this geometrysubstantially no degradation of the resolution of the image is obtainedfrom the center of the converter screen 8 toward the outer perimeter ofthe converter screen 8. This contrasts markedly with the geometry andresolution obtained in the prior art, as shown in FIG. 2. In the priorart structure, the electron exit aperture 19 in the third electrodestructure 16 was axially spaced from the electron entrance aperture 21of the anode electrode 12'. As a result, the electron optics were lessthan optimum resulting in a degradation of the resolution of the imageobtained from the image converter screen 8 near the outer perimeter ofthe image.

An operating potential, as of about 2.5 kV positive with respect tocathode, is applied to the third electrode structure 16 via lead 22which is tapped off the potential source 13.

A second cylindrical focus electrode structure 23 is disposed upstreamof the third electrode 16. The second electrode structure 23 includes ahollow cylindrical member having an electron entrance aperture 24 at theupstream end thereof and an electron exit aperture 25 at the downstreamend thereof. The mouth 17 of the third electrode structure 16 isdisposed within the electron exit aperture 25 of the second electrodestructure 23 to form a two cylinder electron focusing lens. Electrode 23is supported from the third electrode 16 via a plurality of insulatorstmctures 26 disposed about the perimeter of electrodes 23 and 16. Anoperating potential as of about 800 volts positive with respect to thecathode is applied to the second electrode 23 via lead 27 which istapped off the potential source 13.

A first hollow cylindrical focus electrode structure 28 is disposedupstream from the second electrode 23. The first electrode 28 isconveniently formed by depositing a conductive metal, as of aluminum,onto the inside wall of the envelope 2 and is joined to the face plate 3via mating metallic frames 30 as of Kovar. The first electrode 28 has anelectron entrance aperture 29 at the upstream end thereof and anelectron exit aperture 31 at the downstream end thereof, such entranceand exit apertures being of approximately the same size. The electronentrance aperture 24 of the second electrode 23 is disposed within theelectron exit aperture 31 of the first electrode 28 to form a twocylinder electron focusing lens. A suitable operating potential as ofabout 240 volts positive with respect to the cathode is applied to thefirst electrode 28 via lead 32 tapped off the source of potential 13.

The conductive electrode which is mounted within the domed face plate 3includes a cylindrical extension portion 5' to facilitate properfocusing of the electron images emitted from the photocathode 7. Thecylindrical extension 5 is coaxially spaced within the mouth 29 of thefirst cylindrical focus electrode structure 28 to form a two cylinderelectron focus lens.

It has been found that when the various electrode structures within thetube 1 have the following relative dimensions normalized to the maximumdiameter of the first focusing electrode, d, that optimum resolution isobtained over substantially the entire surface of the image converterscreen 8. More particularly, a resolution of 70 line pairs per inch atthe input or 28 line pairs per millimeter on the output screen 8 isreadily obtained. The optimum normalized dimensions are: anode electrodeapertures 21 of 0.12d diameter and an anode length of 0.19:1, thirdelectrode 16 having an entrance aperture of 0.45:! in diameter at themouth which is constricted to 0.27d in diameter midway along the lengthwith the length of the mouth portion being 0.13d and the constrictedmidportion being 0.22d in length, the second electrode 23 having acentral aperture 0.60d in diameter and 0.29d in length, and the firstelectrode 28 having a central aperture 1.0d in diameter and 0.5311 inlength with the axial spacing from the center of the face plate 3 to theend of the cylindrical skirt 5 being 0.24d. For the above relativedimensions, optimum resolution was obtained when the potentials appliedto the various electrodes were as follows, where A is the potentialapplied between the photocathode 7 and the image converter screen 8:anode potential A, third electrode 16 potential 0.1A, second electrode23 potential 0.03A, and first electrode 28 potential 0.0lA.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a minifying image tube, means forming a curved photoemitterelectrode for receiving a photon image and converting same into anelectron image, means forming a flat fluorescent image converter screenof relatively small dimensions relative to said photoemitter forbombardment by the electron image emitted from said photoemitter and forconverting the electron image into an optical image of reduced sizecompared to the size of the electron image as emitted from saidphotoemitter, means forming an electron accelerating and focusingelectrode structure disposed between said photoemitter and saidconverter screen for accelerating the electron image to relatively highvelocity and for focusing same onto said flat image converter screen,said electron focusing and accelerating electrode structure including aplurality of coaxially aligned axially spaced centrally aperturedelectrodes insulatively supported relative to each other to permitindependent operating potentials to be applied thereto in use,successive ones of said electrodes taken in the direction from saidphotoemitter toward said converter screen having successively smallercentral apertures through which the electron image is accelerated andfocused, each of said central apertures having a mouth portion facingthe photoemitter and an exit portion facing said converter screen, theimprovement wherein, each of said focusing accelerating electrodes iscylindrical with the mouth portions of each successively smallerelectrode being disposed within the exit portion of the next precedinglarger electrode to define a series of convergent electron focusinglenses, and said electrode having the smallest central apertureconstitutes an anode and is disposed adjacent said converter screen withthe mouth portion of its central aperture being disposed within theexist portion of the next preceding one of said centrally aperturedfocusing and accelerating electrodes, whereby both electrodes have ageometry so that the equipotentials at the mouth of said anode areshaped to obtain near uniform resolution of the electron image focusedupon said flat converter screen over essentially all parts of saidconverter screen.

2. The apparatus of claim 1 wherein the central aperture of saidelectrode immediately preceding said anode has an exit portion withdimensions constricted substantially relative to the dimensions of itsmouth portion.

3. The apparatus of claim 2 wherein said central apertures in saidelectrodes have approximately the following relative dimensions where dis the maximum diameter of the first upstream focusing electrode, anodeelectrode apertures 0. 12d in diameter and 0.19d in length, nextpreceding upstream electrode from said anode having an aperture 0.45d indiameter at the mouth constricted to 0.27d in diameter midway along itslength with the length of the mouth portion being 0.13d and theconstricted mid portion being 0.22d in length, the second upstreamelectrode from said anode having a central aperture 0.60d in diameterand 029d in length, and the first upstream one of said electrodes havinga central aperture 1.0d in diameter and 0.53d in length.

4. The apparatus of claim 3 including means for producing and applyingthe following potentials to said electrodes where A is the potentialapplied between said photoemitter and said converter screen: A potentialto said anode electrode, 0.1A potential to said next preceding electrodefrom said anode, 0.03A to said second electrode upstream from mid anodeand 0.01A to said first electrode.

1. In a minifying image tube, means forming a curved photoemitterelectrode for receiving a photon image and converting same into anelectron image, means forming a flat fluorescent image converter screenof relatively small dimensions relative to said photoemitter forbombardment by the electron image emitted from said photoemitter and forconverting the electron image into an optical image of reduced sizecompared to the size of the electron image as emitted from saidphotoemitter, means forming an electron accelerating and focusingelectrode structure disposed between said photoemitter and saidconverter screen for accelerating the electron image to relatively highvelocity and for focusing same onto said flat image converter screen,said electron focusing and accelerating electrode structure including aplurality of coaxially aligned axially spaced centrally aperturedelectrodes insulatively supported relative to each other to permitindependent operating potentials to be applied thereto in use,successive ones of said electrodes taken in the direction from saidphotoemitter toward said converter screen having successively smallercentral apertures through which the electron image is accelerated andfocused, each of said central apertures having a mouth portion facingthe photoemitter and an exit portion facing said converter screen, theimprovement wherein, each of said focusing accelerating electrodes iscylindrical with the mouth portions of each successively smallerelectrode being disposed within the exit portion of the next precedinglarger electrode to define a series of convergent electron focusinglenses, and said electrode having the smallest central apertureconstitutes an anode and is disposed adjacent said converter screen withthe mouth portion of its central aperture being disposed within theexist portion of the next preceding one of said centrally aperturedfocusing and accelerating electrodes, whereby both electrodes have ageometry so that the equipotentials at the mouth of said anode areshaped to obtain near uniform resolution of the electron image focusedupon said flat converter screen over essentially all parts of saidconverter screen.
 2. The apparatus of claim 1 wherein the centralaperture of said electrode immediately preceding said anode has an exitportion with dimensions constricted substantially relative to thedimensions of its mouth portion.
 3. The apparatus of claim 2 whereinsaid central apertures in said electrodes have approximately thefollowing relative dimensions where d is the maximum diameter of thefirst upstream focusing electrode, anode electrode apertures 0.12d indiameter and 0.19d in length, next preceding upstream electrode fromsaid anode having an aperture 0.45d in diameter at the mouth constrictedto 0.27d in diameter midway along its length with the length of themouth portion being 0.13d and the constricted mid portion being 0.22d inlength, the second upstream electrode from said anode having a centralaperture 0.60d in diameter and 0.29d in length, and the first upstreamone of said electrodes having a central aperture 1.0d in diameter and0.53d in length.
 4. The apparatus of claim 3 including means forproducing and applying the following potentials to said electrodes whereA is the potential applied between said photoemitter and said converterscreen: A potential to said anode electrode, 0.1A potential to said nextpreceding electrode from said anode, 0.03A to said second electrodeupstream from mid anode and 0.01A to said first electrode.